1. Field of the Invention
The present invention relates to a zoom lens system, in particular, a zoom lens system favorable for an image taking optical system of an image-taking apparatus, such as a video camera or digital still camera, etc.
2. Description of the Related Art
In recent years, video cameras, digital still cameras, and other image-taking apparatuses that use solid-state image pickup elements have become more advanced in functions, and in accompaniment, zoom lenses that are compact and yet high in definition are being demanded as image taking optical systems for such apparatuses.
As a zoom lens that satisfies such needs, there is known a so-called rear focus type zoom lens, which comprises, in the order from the object side, the four lens units of a first lens unit with a positive refractive power, a second lens unit with a negative refractive power, a third lens unit with a positive refractive power, and a fourth lens unit with a positive refractive power, and wherein zooming is performed by moving the second lens unit and focusing and correction of the variation of the image surface that accompanies zooming are performed by using the fourth lens unit (for example, Patent Documents 1 to 6).
Generally in comparison with a zoom lens with which focusing is performed by moving the first lens unit, the effective diameter of the first lens unit is small and the lens system as a whole can be made compact readily in a rear focus type zoom lens. Also, close-distance image taking is enabled, and furthermore, since comparatively compact and lightweight lens units are moved and a small driving force is thus sufficient for driving the lens units, the characteristic that rapid focusing can be performed in the autofocus process is provided. However, when attempts are made to maintain compactness while making the zoom ratio high with such an optical system, it becomes difficult to correct the variation of the chromatic aberration of magnification which occurs during zooming.
Meanwhile, there are known zoom lenses, with which the second lens unit has a negative lens positioned at the most image side thereof and is configured as a whole with three negative lenses and one positive lens for correction of the variation of the chromatic aberration of magnification which occurs during zooming (for example, Patent Documents 6 and 7).
There are also known zoom lenses, with which a high-dispersion glass material with an Abbe's number νd of approximately 21 is used as the material of a positive lens of the second lens unit (for example, Patent Documents 8 and 9).
Also, as a vibration control optical system having a function of repressing the blurring of a taken image, there has been known since priorly a four-unit zoom lens arrangement, comprising first, second, third, and fourth lens units with positive, negative, positive, and positive refractive powers, respectively, wherein the third lens unit is configured from two lens units of positive and negative refractive powers and the lens unit with the positive refractive power in the third lens unit is vibrated to correct the blurring of the image (for example, Patent Document 10). There is also known a four-unit zoom lens arrangement, comprising first, second, third, and fourth lens units with positive, negative, positive, and positive refractive powers, respectively, wherein the entirety of the third lens unit is vibrated to correct the blurring of the image (for example, Patent Document 11).
There is also known a four-unit zoom lens arrangement, comprising first, second, third, and fourth lens units with positive, negative, positive, and positive refractive powers, respectively, wherein the first, second, and fourth lens units and a stop are moved to perform zooming and the entirety of the third lens unit is vibrated to correct the blurring of the image (for example, Patent Document 12).
[Patent Document 1]
    Japanese Patent Application Laid-Open No. H6(1994)-34882 (corresponds to U.S. Pat. No. 5,424,869)[Patent Document 2]    Japanese Patent Application Laid-Open No. H8(1996)-292369 (corresponds to U.S. Pat. No. 5,940,221)[Patent Document 3]    Japanese Patent Application Laid-Open No. H11(1999)-305124 (corresponds to U.S. Pat. No. 6,166,864)[Patent Document 4]    Japanese Patent Application Laid-Open No. H4(1992)-43311 (corresponds to U.S. Pat. No. 5,189,558)[Patent Document 5]    Japanese Patent Application Laid-Open No. H5(1993)-72475 (corresponds to U.S. Pat. No. 5,267,082)[Patent Document 6]    Japanese Patent Application Laid-Open No. H8(1996)-82743[Patent Document 7]    Japanese Patent Application Laid-Open No. 2000-89116 (corresponds to U.S. Pat. No. 6,118,593)[Patent Document 8]    Japanese Patent Application Laid-Open No. H8(1996)-160299 (corresponds to U.S. Pat. No. 5,719,708)[Patent Document 9]    Japanese Patent Application Laid-Open No. 2000-121941[Patent Document 10]    Japanese Patent Application Laid-Open No. H7(1995)-128619[Patent Document 11]    Japanese Patent Application Laid-Open No. H7(1995)-199124 (corresponds to U.S. Pat. No. 5,585,966)[Patent Document 12]    Japanese Patent Application Laid-Open No. 2001-66500 (corresponds to U.S. Pat. No. 6,414,800)
With the making of image-taking apparatuses compact and the making of image pickup elements high in the number of pixels, lens systems that are high in optical performance and yet are compact as a whole are being desired. Also, the recording of still images of high quality is desired even with video cameras.
With a zoom lens, when a rear focus method, with which focusing by using a lens unit besides a first lens unit, is employed, though the lens system as a whole can be made compact, the aberration variations that occur during focusing become large and it becomes difficult to obtain high optical performance with respect to object distances from an object at infinity to an object at close distance.
With the arrangement of Patent Document 6, though a merit is provided in terms of aberration corrections due to the second lens unit being configured from four lenses, since the number of lenses is four, the total lens length tends to become long correspondingly.
Also, with the arrangement of Patent Document 8, though the number of lenses of the second lens unit is small, the number of lenses is large for the optical system as a whole and the lens system as a whole tends to become large. Also, with the arrangement of Patent Document 9, since the second lens unit is configured as an adhered lens of a positive lens and a negative lens, the degree of freedom of design tends to be low.
Meanwhile, with a vibration control optical system, with which a part of the lenses of an image taking system is decentered in parallel in a direction perpendicular to the optical axis, a still image can be obtained even if hand-shake, etc. occurs. However, in the vibration control process, the amount of decentration aberration that occurs becomes large and the optical performance is lowered.